Electric heating elements are used for a number of industrial equipment applications including ovens, furnaces, heat sealers, etc. For some industrial processes it is important to be able to accurately control the chamber temperature of the equipment, which can be accomplished by precisely controlling the electric power applied to the heating elements. For example, SCR power controllers are commonly used for such a purpose.
A SCR power controller uses a silicon controlled rectifier (SCR) or “thyristor” to turn on and off the power applied to the heating elements(s). A SCR is a semiconductor device with at least four layers of alternating N-type and P-type materials which acts as a bistable switch. An SCR conducts when it is forward biased and does not conduct when it is reverse biased.
A zero-crossing switched (zero-switched or burst-fired) SCR power controller works by triggering at the moment when the value of an alternating current (AC) sine wave is at the baseline or “zero” voltage point. The power can be controlled by controlling the number of cycles “ON” to the number of cycles “OFF.” The output will vary from a few cycles ON and a large number of cycles OFF at low input, through half the cycles ON and half OFF at half input, to all cycles ON at maximum input.
FIG. 1 illustrates a first example of a burst-fired current 10. In this example, there is a single AC cycle in a period when the current is ON and then a subsequent period when the current is OFF. The ON and OFF periods, in this example, define a burst-fired current cycle (“CYCLE”), and one or more burst-fired current cycles can create a pattern. The power output is equal to PMAX*ON/CYCLE, where PMAX is the maximum power that can be provided to the heating element. The ratio “ON/CYCLE”, which ranges from zero to one, is often referred as the “duty cycle.”
FIG. 2 illustrates another example of a burst-fired current 12. Unlike the example of FIG. 1, in this example the ON period includes multiple AC cycles. Also, in this example, the burst-fired current cycle (comprising the ON period and the OFF period) and burst-fired current pattern are indicated to be the same, although in other embodiments the burst-fired current pattern may be more than one burst-fired current cycle. Again, the power output is proportional to the ratio PMAX*ON/CYCLE, where PMAX is the maximum power that can be provided to the heating element.
FIG. 3 illustrates an industrial control apparatus 14 including an SCR power controller 16 and a heating element 18. Typically, a power source 20, such as a single-phase line power source in the range of 100-240 AC volts is used. The SCR power controller 16 coverts the AC current provided by power source 20 (typically 60 Hz in the U.S.) into a burst-fired current 22 to control the power applied to the heating element 18 and, therefore, the chamber temperature of the equipment with which it is associated. The SCR power controller can be controlled manually or by a feedback loop (e.g. from a control mechanism using a temperature sensor).
Problems can be encountered with certain forms of feedback. For example, temperature sensors can have time-lag delay and can degrade and/or fail over time preventing the desired precise temperature control of industrial equipment. A more direct way of measuring the power consumed by the heating element is desirable.
For the purposes of determining power consumption, a heating element of an industrial oven or the like can be modeled as a simple resistor because the reactive components of the power usage are relatively small. If the AC current is continuous, this model allows a calculation of the average power dissipated by the heating element as Pavg=(VRMS)2/R, where VRMS is the root-mean-square (RMS) of the voltage and R is the resistance of the heating element. However, this will not give a true RMS value for burst-fired currents.
For industrial process control instruments, analog 4-20 and 10-50 mA current loops are often used. For example, with a 4-20 mA current loop, 4 mA represents the lowest end of the range and 20 mA is the highest end of the range. A key advantage of a current loop is that the accuracy of the signal is not affected by voltage drops in the interconnecting wiring and that the loop can provide operating current to the device.